1,809 research outputs found
Micro-crystalline inclusions analysis by PIXE and RBS
A characteristic feature of the nuclear microprobe using a 3 MeV proton beam
is the long range of particles (around 70 \mu m in light matrices). The PIXE
method, with EDS analysis and using the multilayer approach for treating the
X-ray spectrum allows the chemistry of an intra-crystalline inclusion to be
measured, provided the inclusion roof and thickness at the impact point of the
beam (Z and e, respectively) are known (the depth of the inclusion floor is Z +
e). The parameter Z of an inclusion in a mineral can be measured with a
precision of around 1 \mu m using a motorized microscope. However, this value
may significantly depart from Z if the analyzed inclusion has a complex shape.
The parameter e can hardly be measured optically. By using combined RBS and
PIXE measurements, it is possible to obtain the geometrical information needed
for quantitative elemental analysis. This paper will present measurements on
synthetic samples to investigate the advantages of the technique, and also on
natural solid and fluid inclusions in quartz. The influence of the geometrical
parameters will be discussed with regard to the concentration determination by
PIXE. In particular, accuracy of monazite micro-inclusion dating by coupled
PIXE-RBS will be presented
Three-dimensional simulations of rotationally-induced line variability from a Classical T Tauri star with a misaligned magnetic dipole
We present three-dimensional (3-D) simulations of rotationally induced line
variability arising from complex circumstellar environment of classical T Tauri
stars (CTTS) using the results of the 3-D magnetohydrodynamic (MHD) simulations
of Romanova et al., who considered accretion onto a CTTS with a misaligned
dipole magnetic axis with respect to the rotational axis. The density, velocity
and temperature structures of the MHD simulations are mapped on to the
radiative transfer grid, and corresponding line source function and the
observed profiles of neutral hydrogen lines (H-beta, Pa-beta and Br-gamma) are
computed using the Sobolev escape probability method. We study the dependency
of line variability on inclination angles (i) and magnetic axis misalignment
angles (Theta). By comparing our models with the Pa-beta profiles of 42 CTTS
observed by Folha & Emerson, we find that models with a smaller misaligngment
angle (Theta<~15 deg.) are more consistent with the observations which show
that majority of Pa-beta are rather symmetric around the line centre. For a
high inclination system with a small dipole misalignment angle (Theta ~ 15
deg.), only one accretion funnel (on the upper hemisphere) is visible to an
observer at any given rotational phase. This can cause an anti-correlation of
the line equivalent width in the blue wing (v0)
over a half of a rotational period, and a positive correlation over other half.
We find a good overall agreement of the line variability behaviour predicted by
our model and those from observations. (Abridged)Comment: 15 pages, 13 figures. Accepted for publication in MNRAS. A version
with full resolution figures can be downloaded from
http://www.physics.unlv.edu/~rk/preprint/inclined_dipole.pd
Modeling T Tauri Winds from He I 10830 Profiles
The high opacity of He I 10830 makes it an exceptionally sensitive probe of
the inner wind geometry of accreting T Tauri stars. In this line blueshifted
absorption below the continuum results from simple scattering of stellar
photons, a situation which is readily modeled without definite knowledge of the
physical conditions and recourse to multi-level radiative transfer. We present
theoretical line profiles for scattering in two possible wind geometries, a
disk wind and a wind emerging radially from the star, and compare them to
observed He I 10830 profiles from a survey of classical T Tauri stars. The
comparison indicates that subcontinuum blueshifted absorption is characteristic
of disk winds in ~30% of the stars and of stellar winds in ~40%. We further
conclude that for many stars the emission profile of helium likely arises in
stellar winds, increasing the fraction of accreting stars inferred to have
accretion-powered stellar winds to ~60%. Stars with the highest disk accretion
rates are more likely to have stellar wind than disk wind signatures and less
likely to have redshifted absorption from magnetospheric funnel flows. This
suggests the possibility that when accretion rates are high, disks can extend
closer to the star, magnetospheric accretion zones can be reduced in size and
conditions arise that favor radially outflowing stellar winds.Comment: 41 pages, 11 figures. Accepted by Astrophysical Journa
Dynamics of broken symmetry nodal and anti-nodal excitations in Bi_{2} Sr_{2} CaCu_{2} O_{8+\delta} probed by polarized femtosecond spectroscopy
The dynamics of excitations with different symmetry is investigated in the
superconducting (SC) and normal state of the high-temperature superconductor
BiSrCaCuO (Bi2212) using optical pump-probe (Pp)
experiments with different light polarizations at different doping levels. The
observation of distinct selection rules for SC excitations, present in A and B symmetries, and for the PG excitations, present in
A and B symmetries, by the probe and absence of any
dependence on the pump beam polarization leads to the unequivocal conclusion of
the existence of a spontaneous spatial symmetry breaking in the pseudogap (PG)
state
On the joint security of signature and encryption schemes under randomness reuse: efficiency and security amplification
Lecture Notes in Computer Science, 7341We extend the work of Bellare, Boldyreva and Staddon on the systematic analysis of randomness reuse to construct multi-recipient encryption schemes to the case where randomness is reused across different cryptographic primitives. We find that through the additional binding introduced through randomness reuse, one can actually obtain a security amplification with respect to the standard black-box compositions, and achieve a stronger level of security. We introduce stronger notions of security for encryption and signatures, where challenge messages can depend in a restricted way on the random coins used in encryption, and show that two variants of the KEM/DEM paradigm give rise to encryption schemes that meet this enhanced notion of security. We obtain the most efficient signcryption scheme to date that is secure against insider attackers without random oracles.(undefined
UV excess measures of accretion onto young very low-mass stars and brown dwarfs
Low-resolution spectra from 3000-9000 AA of young low-mass stars and brown
dwarfs were obtained with LRIS on Keck I. The excess UV and optical emission
arising in the Balmer and Paschen continua yields mass accretion rates ranging
from 2e-12 to 1e-8 Mo/yr. These results are compared with {\it HST}/STIS
spectra of roughly solar-mass accretors with accretion rates that range from
2e-10 to 5e-8 Mo/yr. The weak photospheric emission from M-dwarfs at <4000 A
leads to a higher contrast between the accretion and photospheric emission
relative to higher-mass counterparts. The mass accretion rates measured here
are systematically 4-7 times larger than those from H-alpha emission line
profiles, with a difference that is consistent with but unlikely to be
explained by the uncertainty in both methods. The accretion luminosity
correlates well with many line luminosities, including high Balmer and many He
I lines. Correlations of the accretion rate with H-alpha 10% width and line
fluxes show a large amount of scatter. Our results and previous accretion rate
measurements suggest that accretion rate is proportional to M^(1.87+/-0.26) for
accretors in the Taurus Molecular Cloud.Comment: 13 pages text, 15 tables, 14 figures. Accepted by Ap
Three-dimensional dust radiative-transfer models: The Pinwheel Nebula of WR104
We present radiative-transfer modelling of the dusty spiral Pinwheel Nebula
observed around the Wolf-Rayet/OB-star binary WR104. The models are based on
the three-dimensional radiative-transfer code TORUS, modified to include an
adaptive mesh that allows us to adequately resolve both the inner spiral turns
(sub-AU scales) and the outer regions of the nebula (distances of 10^4 AU from
the central source). The spiral model provides a good fit to both the spectral
energy distribution and Keck aperture masking interferometry, reproducing both
the maximum entropy recovered images and the visibility curves. We deduce a
dust creation rate of 8+-1 x 10^{-7} solar masses per year, corresponding to
approximately 2% by mass of the carbon produced by the Wolf-Rayet star.
Simultaneous modelling of the imaging and spectral data enables us to constrain
both the opening-angle of the wind-wind collision interface and the dust grain
size. We conclude that the dust grains in the inner part of the Pinwheel nebula
are small (~100A), in agreement with theoretical predictions, although we
cannot rule out the presence of larger grains (~1 micron) further from the
central binary. The opening angle of the wind-wind collision interface appears
to be about 40 degrees, in broad agreement with the wind parameters estimated
for the central binary. We discuss the success and deficiencies of the model,
and the likely benefits of applying similar techniques to the more the more
complex nebulae observed around other WR/O star binaries.Comment: 10 pages, accepted by MNRA
High count rate {\gamma}-ray spectroscopy with LaBr3:Ce scintillation detectors
The applicability of LaBr3:Ce detectors for high count rate {\gamma}-ray
spectroscopy is investigated. A 3"x3" LaBr3:Ce detector is used in a test setup
with radioactive sources to study the dependence of energy resolution and photo
peak efficiency on the overall count rate in the detector. Digitized traces
were recorded using a 500 MHz FADC and analysed with digital signal processing
methods. In addition to standard techniques a pile-up correction method is
applied to the data in order to further improve the high-rate capabilities and
to reduce the losses in efficiency due to signal pile-up. It is shown, that
{\gamma}-ray spectroscopy can be performed with high resolution at count rates
even above 1 MHz and that the performance can be enhanced in the region between
500 kHz and 10 MHz by using pile-up correction techniques
Combinatorial Bounds and Characterizations of Splitting Authentication Codes
We present several generalizations of results for splitting authentication
codes by studying the aspect of multi-fold security. As the two primary
results, we prove a combinatorial lower bound on the number of encoding rules
and a combinatorial characterization of optimal splitting authentication codes
that are multi-fold secure against spoofing attacks. The characterization is
based on a new type of combinatorial designs, which we introduce and for which
basic necessary conditions are given regarding their existence.Comment: 13 pages; to appear in "Cryptography and Communications
- …